From the docs of the strict module:

This generates a compile-time error if you access a variable that wasn't declared via our or use vars, localized via my(), or wasn't fully qualified. Because this is to avoid variable suicide problems and subtle dynamic scoping issues, a merely local() variable isn't good enough. See my in the perlfunc manpage and local in the perlfunc manpage.

        use strict 'vars';
        $X::foo = 1;         # ok, fully qualified
        my $foo = 10;        # ok, my() var
        local $foo = 9;      # blows up

        package Cinna;
        our $bar;                   # Declares $bar in current package
        $bar = 'HgS';               # ok, global declared via pragma
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The local() generated a compile-time error because you just touched a global name without fully qualifying it. Because of their special use by sort(), the variables $a and $b are exempted from this check.